In head boxes of paper making machines, pulp suspension is fed from the head box to the wire through a lip passage. The area weight profile and the fiber orientation profile transversely to the fiber web can be adjusted by leading desired amounts of diluting liquid at desired points along the width of the fiber web. FI patent publication 92229 discloses an arrangement where mixing units are arranged adjacently to each other along the width of the machine. Fiber suspension and diluting liquid, are fed through separate inlets into each mixing unit where they are mixed and discharged through a single outlet of the mixing unit.
It is well known that the suspension flow to the wire cloth has to be even to maintain the quality of the fiber web even. Clogging of any parts of the feeding elements of the machine will be harmful in this respect. In order to avoid clogging, fiber bundles in the liquid flows have to be avoided. Clogging is particularly harmful when the uneven quality of the fiber web results in tearing of the web and expensive stops in the production.
EP publication 633 416 discloses one valve type where clogging is avoided by using a tangential inlet passage which causes the flow to form a turbulence or vortex in the valve. In this prior art valve, the adjusting of the flow is done by means of an axially movable rod after bringing the flow first into a turbulence.